Violent rainstorms are already more frequent, and they will only get worse. Europe’s metropolises are working on ways to protect themselves.

The warmer the air gets, the more moisture it holds. And that can spell disaster for urban infrastructure.

Cities are installing new drainage systems, green roofs and smart sensors to reduce the potential damage.

During the night of June 11th 2018, the Swiss city of Lausanne received as much rain in one hour as it would normally in two weeks. The railway station was flooded, building lifts stopped functioning and sidewalks were destroyed. That cataclysm was hardly an isolated incident, as climate change produces more and more torrential downpours throughout Europe. Every degree by which the air temperature rises results in 7% more moisture, and that eventually comes down in the form of rain. The most dramatic occurrence was in Copenhagen in 2011, where the city suffered not only flooding but the collapse of the police telephone system and damage to roads, railways and its Metro.

The problem will only get worse. A recent study by the Massachusetts Institute of Technology and ETH Zurich showed that a 4°C increase in average global temperatures by 2100 would lead to a 25% increase in the intensity of extreme rainfall in Europe. Says Philippe Drobinski, professor of meteorology and director of the Dynamic Meteorology Laboratory at École Polytechnique in Paris: “A region that currently experiences an extreme meteorological event every 15 years or so should now expect such a phenomenon about every five years.”

Green roofs

Urban areas, where 74% of Europeans live, are particularly vulnerable because asphalt cannot absorb the excess water. As a result, flooding not only damages infrastructure, but it can also spread waste and even disease as sewers overflow and dirty water mixes with flood water. The bill is steep: between 1980 and 2016, flooding already cost European cities more than €200 billion.

According to Karsten Arnbjerg-Nielsen, professor at the Technical University of Denmark’s Department of Environmental Engineering, cities can take several approaches to protect themselves from weather. “The most common solutions involve building efficient sewage systems and installing drainage networks to channel water to areas where it will cause less damage,” he says. “It is also important to consider the risks when making decisions on land use in urban areas.” This means limiting construction in areas that are most vulnerable to flooding.

Arnbjerg-Nielsen also recommends changes in the way buildings are conceived. “More and more floating structures are being built, such as the IBA Dock office complex in Hamburg,” he notes. Many buildings now feature “green” roofs, which, according to a study from the University of Toronto, can absorb 70% of rainwater through their plants. In France, this technique is already widespread: since 2015, rooftops of new buildings in commercial zones must be partially covered in plants.

Floating houses, IJburg (Amsterdam)

The Netherlands are pioneers for protecting their cities against floods.

Turning a public park into a reservoir

Since the devastating rainfall in 2011, Copenhagen has created sustainable urban drainage systems. These systems are made of bumps, embankments or ditches that can redirect water into areas where it causes less damage, like parks and open spaces. Another example to prepare for future flooding can be seen in Enghaveparken, a large public park. In the event of heavy rainfall, a sunken sports field can store up to 24,000 cubic metres of water, or eight times an Olympic size swimming pool, in retention ponds. The space then will drain out in only one day.

London has launched a strategy of improving its sewage system, which can cope only with once-every-30-years deluges. Such events are now more likely every six years. Through various sustainable drainage techniques, like green roofs or rainwater harvesting systems, the surface water flows in the sewer network will be reduced by 25% by 2040.

Technology can also play a role. In 2016 Chicago installed smart sensors to measure the amount of rainwater and air moisture in different neighbourhoods. The city uses the data to decide which areas need greater protection. Another example is Bologna, which is threatened by the rising level of the Po River. The Italian city has developed a mobile app that citizens can use to notify the authorities of water damage.

Most cities understand the potential problems associated with water. A report by the European Environment Agency showed that water management and flood protection are the leading measures taken to adapt urban planning to climate change. And rightly so, says Arnbjerg-Nielsen: “All forecasts show that the rainfall that we currently describe as high risk will be much more frequent in 20 to 40 years.”

Netherlands: masters of the sea

Even when it doesn’t rain, the Netherlands has a problem: half the country is below sea level. And that level could be almost 70 cm higher by 2100. To tackle this threat, the Netherlands has created large protection systems such as Oosterscheldekering, a 9-km long dam built in the late 1980s to protect Zeeland from tidal waves and flooding. In recent years, the country has also used less spectacular but equally effective techniques, such as developing green roofs, floating buildings and farms.

That experience has made the Netherlands the go-to country for cities rethinking how to handle water. About 2,500 companies are active in the sector. Dutch dredging companies generate 40% of industry revenue worldwide. Experts from other low-lying countries, such as the Philippines and Indonesia, regularly visit the Netherlands to pick up ideas for new solutions. After Hurricane Katrina, which devastated New Orleans in 2005, several Dutch companies were involved in building new dikes and dams.